The invasion and impairment of petroleum and gas producing formations by particulate matter is a well known and costly problem in the oil and gas industry. The invasion and the associated depth of penetration of solids particles into the porous media, which results in plugging the pore spaces, has tendered a broad spectrum of explanations; but the phenomenon is far less than completely understood. Consequently, the proposed remedial treatments are not always successful, if at all.
In drilling and producing oil or gas it is necessary to form a borehole or wellbore by drilling into the earth, and to balance the formation pressure with a drilling fluid or "mud." These fluids, or muds, are commonly aqueous liquids within which there is dispersed clays or other colloidal solids materials. A drilling fluid also serves as a lubricant for the bit and drill stem, and as a carrying medium for the cuttings produced by the drill bit. If oil or gas are found and the oil or gas can be produced in commercial quantities the well is completed. Usually a casing is run from the surface downwardly, set and cemented. The hole is drilled to a depth below the producing formation, and the casing is set to a point near the bottom of the hole. The producing formation is sealed off by the production string and cement, and perforations made in the strata so that the oil or gas can flow into the wellbore. Perforations are made through the casing and cement, and these are extended some distance into the producing formation. A small diameter pipe, or tubing, is then placed in the well generally concentric with the casing to carry the oil or gas product to the surface.
The presence of the drilling fluid, or wellbore fluids generally, also assists in the formation of a crust, or mudcake, on the wall of the wellbore and results in the reduction of fluid losses to the surrounding subsurface strata. Unfortunately however, the presence of particulate solids or fines, in the wellbore fluids also results in pluggage of the pore throats in the wall of the producing formation. Pluggage of these openings or passageways will prevent the conveyance of oil or gas to the wellbore for transport to the surface. The presence of particulate solids transported from within a producing formation to the surface wall of the wellbore also provides a mechanism which may account for this type of pluggage.
It is recognized, in any event, that the absolute pressure within an oil or gas producing formation is directly related to the ability, and duration, of the formation to produce oil or gas. A high formation pressure evidences a formation that contains a large volume of gas. Formations that contain large volumes of gas will produce, and continue to produce, oil or gas. Low pressure, on the other hand, manifests a formation where there is very little gas to drive the oil from the formation, or little or no gas to be produced. Wireline formation testers, as a class, are known for lowering from the surface to a subsurface formation to be tested. A tool of this type includes a fluid entry port, or tubular probe cooperatively arranged with a wall-engaging pad, or packer, which is used for isolating the fluid entry port, or tubular probe from the drilling fluid, mud, or wellbore fluids during the test. The tool, in operating position, is stabilized via the packer mechanism within the wellbore with the fluid entry port, or tubular probe, pressed against the wall of the subsurface formation to be tested. Gas, or other fluid, or both, is passed from the tested formation into the fluid entry port, or tubular probe via a flow line to a sample chamber of defined volume and collected while the pressure is measured by a suitable pressure transducer. Measurements are made and the signals electrically transmitted to the surface via leads carried by the cable supporting the tool. Generally, the fluid pressure in the formation at the wall of the wellbore is monitored until equilibrium pressure is reached, and the data is recorded at the surface on analog or digital scales, or both.
These types of tools have generally served satifactorily, though they are not without their shortcomings. Low permeability formations cannot be effectively tested with the known generally standard low flow rate formation testers, or these types of testers consistently fail after some initial successes before a problem develops. These testers, it has been found, fail to make a good fluidic connection with the formation to be tested. This failure, it is believed, is due to damage to the formation by drilling fluids, or mud solids particles may enter deeply into and plug pore spaces so that no fluid flow can occur. The formation itself, on the other hand, may contain some clay particles in the pore space, and, when a large drawdown pressure is imposed on the formation, those particles may move and plug the pore throat; again, preventing the flow of fluid from the formation. Imperviousness at the wellbore surface for a depth of a fraction of an inch to two inches, it is found, is adequate to choke off the flow of fluids from the formation to be tested.